This mod of Scratch (scratch.mit.edu) will add several new features, including an extensive plugin API.
It is currently developed by the Scratch user SJRCS_011.
Potato is a Squeak virtual machine completely written in Java. It is a derivative of JSqueak (http://research.sun.com/projects/JSqueak/) which was developed by Dan Ingalls. There is a blog on the project at http://potatovm.blogspot.com/.
Self is a prototype-based dynamic object-oriented programming language, environment, and virtual machine centered around the principles of simplicity, uniformity, concreteness, and liveness.